In wireless, or cellular, communication networks, it is important that the downlink and uplink frame timing be synchronized between a base station and mobile devices served by the base station. The mobile devices connected to the base station use the same transmit and receive frequencies.
To ensure that there is no interference between the mobile devices, the mobile devices are assigned time slots or sub-channel frequencies depending on the type of multiplexing (e.g., Time Division Duplex (TDD) or Frequency Division Duplex (FDD)). In either case, frame timing must be precisely maintained over radio links between the base station and the mobile devices.
As shown in FIG. 1, timing must be aligned between a Radio Equipment Controller (REC) 10 and a Radio Equipment (RE) 12 such that the first sample of the Downlink (DL) radio frame is transmitted into the air, i.e., reaches an Antenna Reference Point (ARP) 14, at the same time as the REC's transmit Basic Frame Transmit Reference Point (BFN@TRP) 16. Specifically, the following events all happen simultaneously: the REC's downlink and uplink internal frame timing reference (BFN@TRP), downlink frame (e.g., CPRI downlink frame) starts from the REC 10, the uplink frame (e.g., CPRI uplink frame) arrives at the REC 10, the downlink radio frame leaves the ARP, and the uplink radio frame arrives at the ARP. The allowed timing error in the radio is typically 20 nanoseconds (ns). This means that the first sample may reach the ARP 14 at the BFN@TRP 16 with a ±20 ns delay.
On the Uplink (UL) the first sample of the UL radio frame is the one received at the ARP 14 at the BFN@TRP. The allowed timing error in the radio is also 20 ns. This means that the sample marked by the radio as the first in the UL radio frame must have entered the ARP 14 at the BFN@TRP±20 ns.
For DL path delay compensation, the REC 10 advances the DL baseband data such that it arrives at the radio's ARP 14 point precisely when it starts out at the REC's transmit reference point 16 (BFN@TRP). The REC 10 computes the compensation using the measured DL delay to the radio and the radio DL processing delay the REC 10 receives from the radio during Common Public Radio Interface (CPRI) path setup.
For the UL path delay compensation, the radio uses path delay information received from the REC 10 and the radio's internal UL processing delay to advance the CPRI data such that the arrival time of this UL data is aligned with the outbound data. It is up to the radio to provide further internal timing compensation for each carrier and account for variations due to frequency, operating temperature, and component age on both the UL and DL data paths.
During radio production both the DL and UL data paths must be precisely calibrated for timing alignment. The in-equipment delay, or TOFFSET, obtained at production and stored at each radio is then used for the synchronization process. For this to work, a large amount of delay calibration data must be stored in non-volatile memory. A radio must be re-calibrated after factory repairs and this process is complex and time consuming. In addition, with change in frequency, temperature, and component aging, the stored in-equipment delay can change, which results in timing errors. Although the equipment is designed to allow certain timing errors, wide variations can still occur. If such large variations occur, further calibration is required in the field, which is expensive, time consuming, and introduces maintenance problems.
As such, there is a need for an automatic delay calibration technique which eliminates the need to store calibration data with each radio.